Electric motors generally are either AC or DC. Where a constant speed requirement exists, such as when a motor is to be used as a gyro in a turn coordinator, the AC motor has the advantage that by carefully regulating the alternating current frequency, use of a synchronous AC motor assures regulated speed. The major disadvantage of AC motors is that they are expensive. DC motors are not reliable with respect to regulating the speed and thus have limitations for use as gyro motors in turn coordinators. Further, all DC motors must have a commutating device in order to operate.
Heretofore, the commutating device for DC motors has been the major cause of problems. Brushes wear out, jam and spark. These problems are especially prevalent in very small motors which are run on an intermittent basis. An oxide layer formed on the commutator acts as a lubricant and normally extends the brush life. Intermittent service allows the oxide layer to break down, destroying the lubricating effect.
In an effort to avoid the need for brushes and commutator bars, photoelectric and magnetic commutating switching have recently become popular for use in DC motors. Such motors properly equipped with these types of commutating means have the same desirable torque-speed characteristics as a conventional DC motor. However, these new commutation schemes have been and are expensive.
The design of the DC motors themselves has involved the use of laminations for the windings and where the wires are laid into cavities in the laminations, the process is very expensive. In the case of a wound rotor, the wires must be very securely embedded to be sure that centrifugal force will not cause the windings to fly apart. Further, the rotor must be properly balanced to eliminate vibrations and slip rings must be used to connect the windings to the source of power.